Quilava first appeared as a Cyndaquil in Good 'Quil Hunting. On the way to Azalea Town, Ash heard from a Trainer named Koji that the forest he and his friends were traveling through was populated with Cyndaquil. Ash decided to search for one, but he was separated from his friends and chased into a cave by Team Rocket. While running through the cave Ash found Cyndaquil. The two of them were attacked by Team Rocket's mecha, but Cyndaquil unleashed a powerful Flamethrower to defeat it. The mecha began to explode, so Ash threw a Poké Ball to capture Cyndaquil in order to protect it from the blast. Cyndaquil was successfully captured, even though Ash hadn't really intended to do so. Koji then reappeared and challenged Ash to a battle for ownership of Cyndaquil. Even though Misty and Brock told Ash that Cyndaquil was rightfully his and he didn't have to battle for it, Ash accepted the challenge. Cyndaquil, despite having a disadvantage, was able to use its speed and Tackle attack to defeat Koji's Sandslash. Koji then decided to try to steal Cyndaquil anyway in a net, only to get badly burned when the Pokémon used a Flamethrower at him.
Cyndaquil coming down for an aerial attack on Scyther
Cyndaquil's Gym battle debut came only three episodes later in Ash's battle with Bugsy. Ash had been confident that Cyndaquil could provide the firepower needed to take down the Bug-type Gym. Unfortunately, Cyndaquil's couldn't ignite its fiery quills or use fire-type attacks when called upon. Bugsy's Spinarak used this opportunity to land several Poison Stings and then tied Cyndaquil with a String Shot. Ash recalled Cyndaquil, but sent it out again as his last resort against Scyther. Cyndaquil was put at a disadvantage against Scyther's Slash and its use of Swords Dance to deflect Flamethrower. However, it won the battle by leaping over Scyther's head, hitting it from above, where Scyther was unable to dodge.
As a Cyndaquil with its flames ignited
Cyndaquil's sluggish battle skills were overcome in Hot Matches!, where it was outclassed by Miki's Skarmory. Ash trained with Cyndaquil, raising its attack and speed, and it also learned how to ignite the flames on its back at will whilst fending off wild Beedrill. In a rematch, Cyndaquil took down Skarmory with Flamethrower.
In Spell of the Unown, Cyndaquil defeated Lisa's Mankey with a single Flamethrower. It helped Ash and his friends break into the crystal mansion, but was quickly defeated by Entei.
In A Goldenrod Opportunity, Cyndaquil easily took down Whitney's first two Pokémon. However, Whitney's Miltank proved far too powerful and knocked out Cyndaquil with a trio of Rollouts. In the rematch, it lost to Miltank again, but managed to tire it out so that Pikachu could finish the battle. Against Morty in From Ghost to Ghost, it defeated Morty's Gastly with Tackle, since Gastly was still vulnerable to physical attacks from Noctowl's Foresight. Later, Cyndaquil was stuck battling against Haunter after it used Mean Look. It managed to strike with Swift and counter Hypnosis with a Smokescreen, but Haunter plucked Cyndaquil from the safety of the smoke and used Lick. It then tossed it to the ground, eliminating it from the battle.
In a Christmas special, it was seen helping Delibird find his lost presents and helped to build a Snorlax Snowman.
Arguably, its most notable battle was in the episode, Nerves of Steelix! when Cyndaquil took down Jasmine's powerful Steelix. Steelix used Sandstorm to block Flamethrower, and a series of Digs and Iron Tails to attack Cyndaquil. However, Cyndaquil managed to hide in the holes dug by Steelix and took it down with an inferno fed by a sustained Flamethrower. It also defeated Pryce's Dewgong in Nice Pryce, Baby!, but was easily beaten by Pryce's Piloswine's Blizzard, whom it had rescued from its icy tomb in the previous episode.
With Ash, ready to battle
Cyndaquil helped Ash get through the screening processes of the Silver Conference in A Claim to Flame!, and was entered in battle against Jackson at Misty's recommendation, in order to counter his powerful Meganium. However, it didn't get to fight Meganium, losing to Jackson's Shiny Magneton's Thunder Wave and Tri Attack combination.
Ash left Cyndaquil along with the rest of his Johto team at Professor Oak's Laboratory when he wanted to start his journey fresh in the Hoenn region.
Advanced Generation series
During the Showdown at the Oak Corral, Cyndaquil was seen relaxing on the ground near Professor Oak's laboratory building. Cassidy stepped on Cyndaquil while she was approaching the building's window and Butch sprayed it with a fire extinguisher. Cyndaquil was not badly harmed and later helped defeat the two Team Rocket members using a Flamethrower attack commanded by Professor Oak.
It reappeared in The Right Place and the Right Mime, where it greeted Ash's Torkoal.
Diamond & Pearl series
Ash called Cyndaquil back into his party for the Lily of the Valley Conference in An Old Family Blend!. It was separated from Ash along with Bayleef, Totodile, Snorlax and Corphish when Team Rocket attempted to steal them. While trying to find and reunite with Ash, Cyndaquil was attacked by one of Team Rocket's mechas. In the battle against Team Rocket Cyndaquil demonstrated that it had learned the move Flame Wheel during its time at the lab. Despite its new attack, Cyndaquil was having trouble defeating the robot. Cyndaquil then evolved into Quilava and used its new Eruption attack to destroy their mecha.
Ash's Quilava without its flames lit
In the following episode, Ash used Quilava to battle Nando in the first round of the Lily of the Valley Conference. Quilava went up against Nando's powerful Armaldo, and during the battle it was revealed that it had learned Aerial Ace. When Quilava's Aerial Ace and Armaldo's X-Scissor hit each other, Armaldo went down first. Quilava smiled to itself but then fainted, thus both Pokémon were knocked out.
Best Wishes series
Quilava was reunited with Ash in The Dream Continues!, where it and the rest of Ash's Pokémon posed for a group photo with their trainer.